Fossilized seashell stones are called salagrama, and serves as non-anthropomorphic symbol of Vishnu.

Lakshmi-Narasimha Shaligram Shila

Salagrama or Shaligram refers to a fossilized shell used in South Asia as an iconic symbol and reminder of the God Vishnu as the Universal Principle by some Hindus.[1] Shaligrams are usually collected from river-beds or banks such as the Gandaki river in Nepal.[2] They are considered easy to carry and popular in certain traditions of Vaishnavism, as an aniconic representation of the divine. They are typically in the form of spherical, black-coloured Ammonoidfossils of the Devonian-Cretaceous period which existed from 400 to 66 million years ago.

Shaligramas are mostly black coloured stones with marks, and are the fossilized remains of now extinct sea dwelling ammonites. Hence they are found in river beds and other regions that were once underwater, the most popular being the Himalayas and Nepal. Historically, the use of Shaligrama (or Salagrama) Shilas in worship can be traced to the time of Adi Shankara through the latter's works. Specifically, his commentary to the verse 1.6.1 in Taittiriya Upanishad[3][4] and his commentary to the verse 1.3.14 of the Brahma Sutras[5] suggest that the use of Saligrama in the worship of Vishnu has been a well-known Hindu practice. A good number of false shaligrams too remain in circulation. Since they were underwater, the belief that these creatures only ate tulsi leaves is a myth.

A Shaligrama – which has the marks of a shankha, Chakra, gada and padma arranged in a particular order – is worshiped as Keshava. With the change in the order of the four symbols, the name of the Shaligrama stone is also different and the images of such deities also have similar setting of the four symbols. The various orders and names are given for the twenty four permutations. These are well known names, which are the different names by which Lord Vishnu is known in the Hindu pantheon. The various versions of the Saligrama Shilas or stones vis-a-vis the order of the four symbols are:[6][7]

1 The list of ten avatars varies regionally. The two substitutions involve Balarama, Krishna and Buddha is considered the avatar of Vishnu. Krishna is almost always included; in exceptions, he is considered the source of all avatars.